The present invention relates to a process for electrochemically roughening aluminum for use as printing plate supports. In particular, roughening of the aluminum support which is present in an acid and/or salt electrolyte is effected by an alternating current.
Printing plates, used herein to refer to offset-printing plates, usually comprise a support and at least one radiation-sensitive (photosensitive) reproduction coating arranged thereon. The reproduction coating is applied to the support either by the user, in the case of plates which are not pre-coated, or by the industrial manufacturer, in the case of pre-coated plates. Aluminum or an alloy thereof has gained acceptance as a support material in the field of printing plates. In principle, it is possible to use the supports without pretreatment and modification; however, they are generally modified in or on their surfaces, for example, by a mechanical, chemical and/or electrochemical roughening process, sometimes referred to as graining or etching in literature, a chemical or electrochemical oxidation process and/or a treatment with hydrophilizing agents. In modern continuously working high-speed equipment employed by the manufacturers of printing plate supports and/or pre-coated printing plates, a combination of the aforementioned modifying methods is frequently used, particularly a combination of electrochemical roughening and anodic oxidation, optionally followed by a hydrophilizing step. Roughening is, for example, carried out in aqueous acids, such as aqueous solutions of HCl or HNO.sub.3 or in aqueous salt solutions, such as aqueous solutions of NaCl or Al(NO.sub.3).sub.3, using alternating current. The peak-to-valley roughnesses of the roughened surface, which are defined as mean peak-to-valley roughnesses, R.sub.z, are in the range from about 1 to 15 .mu.m, particularly from 2 to 8 .mu.m. The peak-to-valley roughness is determined according to DIN 4768, October 1970, as the arithmetic mean of the individual peak-to-valley roughness values of five mutually adjacent individual measurement lengths.
Roughening is carried out, inter alia, in order to enhance the adhesion of the reproduction coating to the support and to improve the water acceptance of the printing form, which results from irradiating and developing the printing plate. By irradiating and developing, or decoating in the case of electrophotographically working reproduction coatings, the ink-receptive image areas and the water-retaining non-image areas, the latter generally being the uncovered support surface, are produced on the printing plate in the subsequent printing operation, thus producing the actual printing form. The final topography of the aluminum surface which is to be roughened is influenced by various parameters, as is explained, by way of example, in the text which follows.
The paper, "The Alternating Current Etching of Aluminum Lithographic Sheet", by A. J. Dowell, published in Transactions of the Institute of Metal Finishing, 1979, Vol. 57, pages 138 to 144, presents basic comments on the roughening of aluminum in aqueous solutions of hydrochloric acid, based on variations of the following process parameters and an investigation of the corresponding effects. The electrolyte composition is changed during repeated use of the electrolyte, for example, in view of the H.sup.+ (H.sub.3 O.sup.30) ion concentration (measurable by means of the pH) and the Al.sup.3+ ion concentration. As a result of these changes, influences on the surface topography are observed. Temperature variations between 16.degree. C. and 90.degree. C. do not effect changes until temperatures are 50.degree. C. or higher. The effect becoming apparent, for example, as a significant decrease in film formation on the surface. Variations in roughening time between 2 and 25 minutes lead to an increasing metal dissolution with increasing duration of action. Variations in current density between about 2 and 8 A/dm.sup.2 result in higher roughness values with rising current density. If the acid concentration is in the range from about 0.17 to 3.3% of HCl, only negligible changes in pit structure occur between about 0.5 and 2% of HCl. Below 0.5% of HCl, the surface is only locally attacked and at the high values, an irregular dissolution of Al takes place. An addition of SO.sub.4.sup.2- ions or Cl.sup.- ions in the form of salts, e.g., by adding Al.sub.2 (SO.sub.4).sub.3 or NaCl, can also influence the topography of the roughened aluminum. Rectification of the alternating current shows that both half-wave types are necessary to obtain a uniform roughening. The influence of frequency changes or of superpositions of currents of different frequencies are not investigated; a constant frequency of about 50 Hz was utilized.
The influence of the electrolyte composition on the quality of roughening is, for example, also described in the following publications, in which standard alternating current having a frequency from about 50 to 60 Hz is used:
German Offenlegungsschrift No. 2,250,275 (=British Pat. No. 1,400,918) specifies aqueous solutions containing from 1.0 to 1.5% by weight of HNO.sub.3 or from 0.4 to 0.6% by weight of HCl and optionally from 0.4 to 0.6% by weight of H.sub.3 PO.sub.4, for use as electrolytes in the roughening of aluminum for printing plate supports by means of an alternating current,
German Offenlegungsschrift No. 2,810,308 (=U.S. Pat. No. 4,072,589) mentions aqueous solutions containing from about 0.2 to 1.0% by weight of HCl and from 0.8 to 6.0% by weight of HNO.sub.3 as electrolytes in the roughening of aluminum with alternating current,
German Auslegeschrift No. 1,238,049 (=U.S. Pat. No. 3,330,743) mentions protective colloids acting as inhibitors, for example, lignin, benzaldehyde, acetophenone or pine needle oil, as additional components in aqueous HNO.sub.3 solutions used in the roughening of aluminum for printing plate supports with alternating current,
U.S. Pat. No. 3,963,594 specifies aqueous solutions containing HCl and gluconic acid as electrolytes in the electrochemical roughening of aluminum for printing plate supports.
Admittedly, the use of aqueous solutions comprising several components to roughen aluminum may lead to more or less uniformly roughened surfaces, but monitoring the bath composition is very expensive, particularly in the case of the presently preferred continuously working high-speed processing equipment for strips. This measure, however, is necessary in practice, since the composition of the electrolyte often changes in the course of the process.
Another known possibility for improving the uniformity of electrochemical roughening comprises a modification of the type of electric current employed, including, for example,
using an alternating current, in which the anodic voltage and the anodic coulombic input are higher than the cathodic voltage and the cathodic coulombic input, according to German Auslegeschrift No. 2,650,762 (=U.S. Pat. No. 4,087,341), the anodic half-cycle period of the alternating current being generally adjusted to be less than the cathodic half-cycle period. This method is, for example, also referred to in German Offenlegungsschrift No. 2,912,060 (=U.S. Pat. No. 4,301,229), German Offenlegungsschrift No. 3,012,135 (=published UK Patent Application No. 2,047,274) or German Offenlegungsschrift No. 3,030,815 (=U.S. Pat. No. 4,272,342),
using an alternating current, in which the anodic voltage is markedly increased compared with the cathodic voltage, according to German Offenlegungsschrift No. 1,446,026 (=U.S. Pat. No. 3,193,485),
interrupting the current flow for 10 to 120 seconds and re-applying current for 30 to 300 seconds, using alternating current and, as the electrolyte, and aqueous solution of 0.75 to 2.0N HCl, with the addition of NaCl or MgCl.sub.2, according to British Pat. No. 879,768. A similar process comprising an interruption of current flow in the anodic or cathodic phase is also disclosed in German Offenlegungsschrift No. 3,020,420 (=U.S. Pat. No. 4,294,672).
The aforementioned methods may lead to relatively uniformly roughened aluminum surfaces, but each requires a comparatively great equipment expenditure and, in addition, are applicable only within closely limited parameters.